Prevention of microbial growth in metal working fluids

ABSTRACT

A method of preventing microbial growth in oil-water metalworking fluid in a machine wherein lubricating oil or hydraulic or tapping fluid can contaminate the metalworking fluid, comprises adding a partitionable anti-microbial agent, active against micro-organisms present in the metalworking fluid, to the lubricating oil or hydraulic or tapping fluid whereby an effective quantity of anti-microbial agent can migrate into the metalworking fluid to reduce micro-organism activity. 
     A machine tool lubricating oil or hydraulic or tapping fluid composition comprises in a mineral oil or other lubricant base and an oil-water partitionable anti-microbial, active against micro-organisms present in oil-water metalworking fluids.

FIELD OF THE INVENTION

This invention relates to the prevention of microbial growth inmetalworking fluids in a machine, wherein lubricating oil or hydraulicor tapping fluids can contaminate the metalworking fluid and tomachine-tool lubricants, hydraulic or tapping fluids and the likeincorporating anti-microbial agents, and specifically relates to amethod of introducing antimicrobial agents into metalworking fluids.

BACKGROUND TO THE INVENTION

In many mechanical environments, there will be present both oil basedand water based compounds. Many machines use hydraulic powerincorporating a high pressure hydraulic fluid, an oil, in a sealedenvironment. However, seals age and leakage of hydraulic fluid can occurto contaminate metalworking fluid.

Mechanical processes often need a lubricating compound to preventfriction between moving parts. An example of such a system is industrialmetal-working machine tools wherein the work piece and tooling arepositioned and moved on flat bearings generally referred to asslideways. These surfaces require a lubricant—the slideway lubricant—toreduce the metal/metal contact therefore eliminating friction and‘stick-slip’ motion, and prevent wear on the slideways. This isessential to maintain the potentially very expensive machine tool ingood condition and to ensure machining accuracy. As most machiningprocesses also utilise a flood applied, water-oil emulsion basedmetalworking fluid to cool and lubricate the working zone the slidewaylubricant is constantly washed off the surfaces of the slides. To combatthis most machine tools automatically dispense oil onto the slides on acontinuous or metered basis. Given the very low cost of the slidewaylubricant this total loss system is the most cost effective method ofensuring that the performance of the machine is not compromised. Thelost lubricant, tramp oil, is washed with the metalworking fluid into asump, from which the metalworking fluid is recycled.

Whilst such methods ensure that the machine stays in good condition ithas the opposite effect on the metalworking fluids. Being oil-wateremulsions, metalworking fluids provide an ideal environment formicrobial growth. Although great efforts are made when formulatingmetalworking fluids to exclude obvious sources of nutrients and toensure that the fluids are as resistant as possible to microbial growththis is largely negated if contaminated by the slideway lubricant. Sinceslideway lubricants are generally based on refined mineral oils andcontain elements such as sulphur and phosphorus (as anti-wear andanti-corrosion agents), these provide ideal nutrients formicro-organisms. It is accepted throughout the metalworking industrythat one of the main causes of failure for metalworking fluids ismicrobial growth promoted by excessive contamination with slidewaylubricants. In systems with low agitation the slideway oil can separatefrom the emulsion, sealing it from the air. In such anaerobic conditionsmicrobial growth in fluids produces by-products of respiration, such asHydrogen Sulphide which causes an unpleasant odour and makes the systemunpleasant to work with.

Hand applied tapping fluids, may be used for tapping, reaming anddrilling operations, and when used on machine tools with water mix,flood applied cutting fluids can also contaminate the cutting fluid.

Several attempts have been made to overcome this problem, which include:

-   -   i) Use of synthetic, biologically-hard base fluids instead of        mineral oil as the base for slideway lubricants. While this may        reduce the overall level of nutrient for the microbes the key        sulphur and phosphorus agent agents are still present. Synthetic        base fluids are also prohibitively expensive for ‘total loss’        systems.    -   ii) Use of the metalworking fluid concentrate as the slideway        lubricant. Although this overcomes the contamination problem the        primary function as a slideway lubricant is compromised. The        ease with which these products can be washed away can leave        slideways ‘dry’ resulting in high friction and poor control of        the tooling/work piece. The emulsification of this additional        concentrate can also lead to the cutting fluid strength        increasing to excessive levels.    -   iii) Use of oil skimmers/separators. These can vary in both        price and performance. The simplest and cheapest types are        either belt or disc skimmers that are immersed directly into        machine sumps and pick up free oil from the surface of        metalworking fluids. These are limited in their performance due        to the fact that the oil will only separate from the fluid when        there is no agitation (i.e. in ‘dead’ areas of the sump) or when        the fluid is saturated with oil. At the other end of the scale        are stand alone machines that extract fluid from machine sumps        and remove any oil contamination. These can either be static,        dedicated to a single machine, or mobile to service multiple        machines on a rota basis. Although they are effective at        removing the contamination from the fluids the cost of these        units can be tens of thousands of pounds each. This can mean a        significant capital investment even for moderately sized        engineering shops.    -   iv) use of an anti-microbial agent in the cutting fluid,        anti-nicrobials are dangerous chemicals, especially in        concentrated form as an agent, and the anti-microbial needs to        be added in correct dosages to maintain function.

PRIOR ART

The most effective way to prevent microbial growth in metalworkingfluids has been the administration of an anti-microbial compound to thefluid. The addition of anti-microbials to fluids has been common placein both household and industrial environments.

-   Document-A—U.S. Pat. No. 4,968,323 discloses the use of a biocide    composition or preservative in hydrocarbon fuels and metalworking    fluids. The biocides are added to fuels such as home heating oil,    diesel and jet fuels which are commonly stored in tanks where a    layer of water can accumulate under the fuel. Also disclosed is the    addition of bromopinacolone to emulsifiable oils to prevent growth    of bacteria in metalworking fluids such as cutting and rolling    fluids. Addition of the bromopinacolone, before contamination or    after contamination of the metalworking fluid is disclosed.-   Document-A—U.S. Pat. No. 4,414,121 discloses the addition of an    anti-microbial agent to a water-based metal-working lubricant to    prevent microbial growth therein.-   Document-A—U.S. Pat. No. 5,508,417 discloses an broad-spectrum    isothiazole anti-microbial agent which can be used in many    situations to prevent microbial attack, generally this micro biocide    is applied in a carrier such as water, a solvent or the like.-   Document-A—U.S. Pat. No. 4,946,612 discloses a dual function oil    composition for use both as a lubricating oil on sliding surfaces    and as a metalworking fluid to solve the problem of having two    different solutions which can mix to their detriment. A secondary    property of this oil composition is bacterial or fungal resistance.

However, addition of anti microbial compositions into metalworkingfluids can be problematic, most of the compositions used are toxic andcomprise dangerous chemicals. The quantities of anti microbial compoundsused in metalworking fluids must be carefully monitored so that firstly,the concentration of compound is high enough to have a sufficient antimicrobial effect and secondly, that the concentration is not at a levelhigh enough to cause damage to people working in the vicinity ofmachinery treated with it.

OBJECT OF THE INVENTION

It is an object of the invention to overcome the above-stated problem byintroducing an anti-microbial agent to a metalworking fluid in a novel,safe and economic manner.

THE INVENTION

According to the present invention a method of preventing microbialgrowth in an oil-water emulsion metalworking fluid in a machine whereinlubricating oil or hydraulic or tapping fluid can contaminate themetalworking fluid, comprises adding an oil-water partitionableanti-microbial agent to the lubricating oil or hydraulic or tappingfluid whereby an effective quantity of the anti-microbial compound canpartition into the metalworking fluid to reduce micro-organism activity.The lubricating oil or hydraulic or tapping fluid is used as a vehicleto deliver the anti-microbial agent into the metalworking fluid in asafe and effective manner. The agent can either be incorporated into thelubricating oil or hydraulic or tapping fluid or added to thelubricating oil or hydraulic or tapping fluid.

Also according to the present invention, a lubricating oil or hydraulicor tapping fluid composition for use in a machine, where an oil-wateremulsion metalworking fluid is present, comprises a mineral oil base andan oil-water partitionable anti-microbial agent which will partitioninto the metalworking fluid and prevent micro-organism growth in themetalworking fluid particularly if the lubricating oil or hydraulic ortapping fluid seals the metalworking fluid to produce an anaerobicenvironment.

Further according to the method of the present invention, and wherein:

-   -   i) the machine has tooling to machine a work piece;    -   ii) the metalworking fluid is a cutting fluid;    -   iii) the machine tool has reservoir or sump to which cutting        fluid drains and from which cutting fluid is delivered to the        tooling and work piece    -   the method further comprises the step of:    -   v) using migration of lubricating oil or hydraulic or tapping        fluid into the cutting fluid sump as a vehicle to deliver        effective quantities of the anti-microbial agent into the        cutting fluid sump.

The machine tool may have a reservoir for lubricating oil or hydraulicor tapping fluid and the anti-microbial agent may be added to thelubricating oil or hydraulic or tapping fluid in the reservoir; or theanti-microbial agent may be incorporated as a composition with thelubricating oil or hydraulic or tapping fluid prior to use in themachine.

The machine tool may have a slideway and the lubricating oil may be aslideway lubricant.

An anti-microbial agent is herein defined “as any ingredient impartingmicrobial inhibiting properties”.

Slideway lubricants are specialised products normally containing, in thelubricant base, agents for anti-wear, tackiness (to control excessivewashout) and demulsification (to limit harmful effects of emulsifyingthe slideway lubricant into the cutting fluid).

The present invention thus uses a slideway lubricant that incorporatesan anti-microbial agent which, when the lubricant is washed into ametalworking fluid, tramp oil, will transfer into the aqueous phasewhere it will maintain anti-microbial properties in the fluid. All ofthe primary functions of the slideway lubricant (lubrication, anti-wear,etc) are unaffected by the inclusion of the agent and no specialistequipment is required, the lubricant is used in the existing lubricationsystem on each machine.

This method of adding to the microbial resistance of the fluid is also‘intelligent’ in that the more tramp oil collected in the cutting fluidsump, the more anti-microbial agent will be transferred into the fluidmaximising its resistance. In cleaner systems where less tramp oilcollects there is less requirement for high levels of extraanti-microbial agent.

EXAMPLE

The anti-microbial agent for inclusion in the hydraulic or tapping fluidor slideway lubricant can be any of the commercially availablemicrobicides which has a good degree of solubility in both oil based andwater based system that would be familiar to anyone in either themetalworking or biocide industries. Examples include formaldehydereleasing compounds such as triazine derivatives and oxazolidines, ornon-formaldehyde products such as benzisothiazolinones and parachlorometacresol.

Factors in choosing the anti-microbial agent are:

-   -   i) Compatibility with the hydraulic or tapping fluid or slideway        lubricant. The agent should not interfere with the primary        function of the hydraulic or tapping fluid or slideway        lubricant.    -   ii) Balanced oil and water solubility. The agent should be        soluble in the slideway lubricant but also be sufficiently        soluble in water to ensure that it is transferred into the        metalworking fluid.    -   iii) Broad spectrum of anti-microbial activity. Metalworking        fluids can be contaminated by bacteria and fungi both of which        can have deleterious effects.    -   iv) Good toxicological profile. Metalworking fluids commonly        come into contact with skin therefore the anti-microbial agent        should pose as little hazard as possible.

Following these criteria, the preferred anti-microbial compound for theinvention is 7a-ethyldihydro-1H,3H,5H-oxazolo(3,4-c)oxazole. This iscommercially available from Angus Chemie under their trade name BiobanCS-1246. The properties of articular interest for this agent are:

-   -   i) No detrimental effect on the lubrication characteristics of        the slideway lubricant. Experimental detail for specific        examples is given later.    -   ii) Octanol/water partition co-efficient (log Pow) of 0.28.        Indicating primarily oil solubility but with adequate water        solubility to allow transfer of the anti-microbial into the        water phase.    -   iii) Broad spectrum of anti-microbial activity indicated by the        minimum inhibitory concentrations (MIC) for common spoilage        organisms as follows:

Organism MIC (parts per million) Bacterial: Enterobacter aerogenes250-300 Escherischia coli 450-500 Pseudomonas aeruginosa 800-850Staphylococcus aureus 200-250 Fungal: Aspergillus niger  65-125 Fusariummoniliforme 125-250 Saccharomyces cerevisiae 16-33

-   -   iv) Low toxicity. As supplied the anti-microbial is harmful by        inhalation and in contact with the skin, irritating to eyes and        skin but is not a sensitising agent and is non-mutagenic.        However at typical use dilutions there is no hazard with the        product. This is indicated by its approval in Europe as a        cosmetic preservative up to levels of 3000 parts per million.

The experimental details referred to above are:

Four samples consisting of a mineral oil base, a commercially availableslideway lubricant agent package (containing agent agents for anti-wear,corrosion inhibition etc), a tackiness agent (to promote adhesion of theslideway lubricant to metal surfaces) and anti-bacterial agent (BiobanCS1246) were prepared as follows.

[Note: All compositions are % weight/weight.]

Sample A Sample B Sample C Sample D 500 Solvent neutral¹ 60.45 62.9566.45 67.95 150 Solvent neutral² 36.30 32.80 28.30 25.80 Hitec 510³ 2.252.25 2.25 2.25 Hitec E151⁴ 1.00 1.00 1.00 1.00 Bioban CS1246 0.00 1.002.00 3.00 ¹paraffinic mineral oil with a kinematic viscosity ofapproximately 100 cSt at 40° C. ²paraffinic mineral oil with a kinematicviscosity of approximately 30 cSt at 40° C. ³slideway lubricant agentpackage commercially available from Ethyl Petroleum Agents Ltd.⁴polyisobutylene tackifier commercially available from Ethyl PetroleumAgents Ltd.

These samples were then subjected to an anti-bacterial screening testagainst Pseudomonas aeruginosa—one of the most common spoilage organismsfound in cutting fluids.

The oil samples were placed in a 10 mm diameter “well” in the centre ofan inoculated agar plate. The plates were then incubated for 48 hoursand visually inspected. Any anti-bacterial activity of the samples isshown as a zone of inhibition—zero bacterial growth—around the original10 mm diameter well, the diameter of which can be measured to give asemi-quantitative estimate of activity.

Sample Diameter of zero growth Inhibited zone A 10 mm  0 mm B 14 mm  4mm C 20 mm 10 mm D 22 mm 12 mm

[Note: To obtain the measure of anti-bacterial activity the originaldiameter of the “well” must be subtracted (10 mm is therefore zeroinhibition).]

From these results it is evident that the most cost effective balancebetween biocide level and activity was reached with sample C.

One of the key criteria in selecting an anti-microbial is to ensurecompatibility with the slideway lubricant. Two of the key measures ofthis are the anti-wear and demulsification properties of the oil. SampleA (untreated slideway lubricant) and Sample C were tested using industrystandard test methods as follows:

Demulsification test (40 mls of oil and 40 mls of water are mixedtogether in 100 ml measuring cylinder. At 5 minute intervals the volumeof water separated from the oil/water mix is recorded).

Sample A Sample C Volume of H₂O separated from Time (minutes) oil/watermix (mls) 0 0 0 5 3 7 10 15 8 15 16 9 20 17 10 25 20 15 30 30 15 35 3618 40 37 24 45 37 29 50 38 31 55 38 32 60 39 34

Although the rate of separation of water is slowed by the inclusion ofthe anti-microbial, demulsification does still occur. In practice theslightly slower rate of separation should provide a longer time scalefor the transfer of the anti-microbial into the cutting fluid.

The shell 4 ball wear test was carried out to determine the twolubricants ability to prevent wear.

Shell 4-ball ear test (60 kg load run for 15 minutes) Sample A

Ball number Wear scar dimensions mm 1 0.709 × 0.723 2 0.728 × 0.709 30.708 × 0.725 Mean scar diameter = 0.717 mm

Sample C

Ball number Wear scar dimensions mm 1 0.479 × 0.457 2 0.463 × 0.444 30.470 × 0.451 Mean scar diameter = 0.462 mm

It can be seen that anti-wear properties are improved by the inclusionof an anti-microbial agent, despite the overall composition containingless paraffinic mineral oil. The improvement is unexpected and, whilenot fully explained, may be due to catalysation of chemical reactionbetween the sulphur and phosphorus anti-wear agent agents present in theoil and the steel surface.

1. A slideway lubricant composition for metal-working machineryutilizing metalworking fluids, comprising a slideway lubricating oil,having a paraffinic mineral oil, an anti-wear agent, a polyisobutylenetackifier, a corrosion inhibitor, and a demulsification agent incombination with 7a-ethyldihydro-1H,3H,5H-oxazolo(3,4-c)oxazole as anoil-water partitionable anti-microbial agent compatible with and solublein said slideway lubricating oil and of sufficient solubility in waterfor transference into a metalworking fluid.